Many industrial processes require precise monitoring of the true temperature, i.e., the thermodynamic temperature, of a material being processed. In some processes, temperature deviations indicate a process failure, in others, changes in temperature and/or emissivity indicate the progression of the monitored process. Examples of processes which require precise temperature monitoring include: metal refining and working, glass and glass product manufacture, firing ceramics, integrated circuit fabrication, electricity generation, chemical and pharmaceutical manufacture and many other industrial processes.
It should be appreciated, that in many cases it is not practical to measure the temperature of the processed material directly, such as by thermocouple, due to the delicacy of the process, the reaction speed of the temperature measurement device or the environmental conditions. Rather, the temperature material. Typically, these emissions are not directly detected, because they must pass through an intervening medium, such as hot air or smoke, which usually has an unknown and variable transmission spectrum.
Several non-contact temperature measurement methods are known in the art. Brightness pyrometry is a general name for temperature measurement methods which determine the temperature of an object based on the total amount of radiation emitted by the object. Typically, only the radiation emitted in a certain spatial direction and within a certain wavelength range is measured. Brightness pyrometry is further described in "Traite de Pyrometric Optique", by C. Ribaud, Paris, 1931.
A major limitation of brightness pyrometry is that the determined temperature is correct only if the product of the emissivity (.epsilon.) of the object being measured and the emissivity (.epsilon.) of the object being measured and the transmission spectrum of the intervening medium (.tau.), is known at each measurement time.
Color-ratio pyrometry is a general name for temperature determination methods which determine the temperature based on the ratio between the intensity of light emitted at first wavelength and the intensity of light emitted at a second wavelength. This pyrometric technique is further described in "Traite de Pyrometric Optique", cited above, in "Optishe Pyrometrie", by F. Hoffman and C. Tingwaldt, published by Braunschweig, 1938 and in "Some Consideration of Error of Brightness and Two-Color Types Spectral Radiation Pyrometers", by E. S. Pyatt, in British Applied Physics, Vol. 15, No. 5, pp. 264-268, 1954.
High accuracy of temperature determination using the color-ratio pyrometric method can only be achieved in cases where the product of the emissivity and the medium transmission spectrum is constant or gray and does not change with respect to time or wavelength. The value of the product does not need to be known. In order to overcome this limitation, an advanced method, described in "High Speed Radio Pyrometry", by G. A. Hornbeck, in a symposium on "Temperature, its Measurement and Control in Science and Industry", Vol. 3, p. 2425, New York, 1962, "A Review of Multicolor Pyrometry for Temperatures Below 1500.degree. C.", by P. M. Reynolds, in British Applied Physics, Vol. 15 pp. 579-589, 1964, and in "Measurement True Temperature Real-Bodies, Methods and Apparatus Optical Pyrometry", by E. D. Glazman and I. I. Novikov in Science 1983, pp. 21-27, Moskva 1983 (in Russian), uses the ratios of the intensities of three or four wavelengths. In the three wavelength method, the requirement for a high accuracy of the result is that .epsilon.(.lambda..sub.1,T)*.epsilon.(.lambda..sub.3,T)=.epsilon..sup.2 (.sub.2,T), where .lambda..sub.i is the wavelength and T is the true temperature of the object. In the four wavelength method, the requirement for a high accuracy of the results is that .epsilon.(.lambda..sub.1,T) *.epsilon.(.lambda..sub.4,T)=.epsilon.(.lambda..sub.2,T)*.epsilon.(.lambda ..sub.3,T).
Another pyrometric method, multi-wavelength pyrometry, is a general name for temperature measurement methods which:
(a) determine the apparent temperature at several wavelengths using one of the abovedescribed pyrometric methods; and
(b) estimate the true temperature based on a model of the dependency of emissivity on wavelength.
Multi-wavelength techniques are further described in "Determination of Emissivity of a Substance from the Spectrum of its Thermal Radiation and Optimal Methods of Optical Pyrometry", by D. Y. Swet, in "High Temperatures-High Pressures", Vol. 8, pp. 493-498, 1976, "Multi-Wavelength Pyrometry", by P. B. Coates, in Metrology, No. 17, pp. 103-109, 1981 and "Noncontact Temperature Measurement 1, Interpolation Based Techniques", by M. A. Khan, C. Allemand and T. W. Eagar, in Rev. Sci. Instrum., 62(2), pp. 392-402, 1991 the disclosures of which are incorporated herein be reference.
A limitation of multi-wavelength pyrometric techniques is that in many cases the differences between the measured temperatures can be explained by more than one emissivity model, so the true temperature cannot be determined without a correct model. It should be noted that in several applications satisfactory results have been achieved using a simple model.
Some approximation methods suitable for multi-wavelength pyrometry are described in "Determination of Emissivity", "Multi-Wavelength Pyrometry" and "Noncontact Temperature Measurement", cited above. However, these methods are only useful when the dependence of the emissivity on the wavelength is generally known. Otherwise, the error in the true temperature determination may very well be larger than the error in true temperature determination using one of the first two abovedescribed pyrometric techniques.
A major limitation of most known pyrometric techniques is their inability to deal with changes in emissivity, in particular, where such changes cannot be anticipated in advance.